Energy Aware Real Time SystemsIntroductionSlide 3Important Facts (1)Workload ProfileImportant Facts (2)Variable Voltage ProcessorsCrusoe processorDynamic Voltage Scaling (DVS)Simple DVS-SchemeDVS-exampleEnergy-Time TradeoffsSimple DVS scheme handling RT-taskSlide 14Energy aware scheduling in RT SystemsEnergy aware RTS TechniquesReal Time - DVS schemesOS level energy management: Inter-task EDFStatic Voltage Scaling EDF: MotivationStatic Voltage Scaling EDF: exploiting holesStatic Voltage Scaling EDFStatic voltage scaling: ExampleSlide 23Slide 24What if (Ci < WCi )??Slide 26What if (Ci < WCi )?? (contd..)Cycle conserving EDF: ExampleSlide 29Slide 30Slide 31Cycle Conserving EDFLook-Ahead EDFRelative performanceCompiler Level Energy ManagementCompiler level energy optimization frameworkIntra-task Voltage Scheduling (Why..)Intra-task Voltage Scheduling (What..)Slide 39Simple Intra-task DVS: exampleSlide 41Slide 42ConclusionsSlide 44ReferencesCprE 458/558: Real-Time Systems (G. Manimaran) 1Energy Aware Real Time SystemsG. Sudha Anil KumarReal Time Computing and Networking LaboratoryDepartment of Electrical and Computer EngineeringIowa State UniversityCprE 458/558: Real-Time Systems (G. Manimaran) 2Introduction•Energy consumption is an important issue in embedded systems. –Mobile and portable devices.–Laptops, PDAs.–Mobile and Intelligent systems: Digital camcorders, cellular phones, and portable medical devices.•Embedded devices play prominent roles in a variety of applications–medical sensors in human body.–Signaling sensors in war fields. •A typical networked embedded system consists of:–Computing subsystem -- driven by an embedded processor operated by a RTOS. –Communication subsystem -- consists of a radio chipset driven by a firmware. Micorprocessor, Digital Signal Processor (DSP)Radio, RF amplifiers, A-to-D & D-to-A cktsA typical Embedded SystemBatteryComputing Subsystem(Driven by RTOS)Communication Subsystem(Driven by Firmware)IntroductionCprE 458/558: Real-Time Systems (G. Manimaran) 4Important Facts (1)•The peak computing rate needed is much higher than the average throughput that must be sustained;•High performance is needed only for a small fraction of time, while for the rest of time, a low-performance, a low-power processor would suffice.CprE 458/558: Real-Time Systems (G. Manimaran) 5Workload ProfileTimeWork loadPeak Computing Rate is neededAverage rate would sufficeCprE 458/558: Real-Time Systems (G. Manimaran) 6Important Facts (2)•Processors are based on CMOS logic-- Static power + Dynamic powerDynamic power (due to switching activity)•P α V2 . f•V α f V: voltage; P: power; E: Energy•E = P * Tcc Tcc = CC/f•Ei = K .cci . f2Where Tcc : execution time; CCi : # clock cycles of task Ti. f : frequency at which Ti is run.CprE 458/558: Real-Time Systems (G. Manimaran) 7Variable Voltage Processors•Modern processors operate at multiple frequency levels.–Crusoe Processor: Transmeta Corporation–PowerNow! Technology: AMD–Intel XScale: Intel•Higher the frequency level higher the energy consumptionCprE 458/558: Real-Time Systems (G. Manimaran) 8Crusoe processor•What is the power consumption of a Crusoe processor? [1]The extremely low power consumption delivered on multimedia applications can be directly attributed to a new feature called LongRun power management. LongRun has the distinct ability to analyze the application workload dynamically and to adjust continuously the processor's speed (MHz) and voltage to provide the necessary performance. This new feature promises to extend the battery life of all applications, most specifically those requiring the constant attention of the processor. This is a dramatic departure from today's ultra-light PCs, which are incapable of delivering over one and a half or two hours of runtime for DVD movies.CprE 458/558: Real-Time Systems (G. Manimaran) 9Dynamic Voltage Scaling (DVS)•DVS scales the operating voltage of the processor along with the frequency. •Since energy is proportional to f2 , DVS can potentially provide significant energy savings through frequency and voltage scaling.CprE 458/558: Real-Time Systems (G. Manimaran) 10Simple DVS-SchemeDVSNext taskOver loadedUnder loadedf = F/2f = FTask queuesystemCprE 458/558: Real-Time Systems (G. Manimaran) 11DVS-example•Consider a task with a computation time 20 units.•Energy of Ti without DVS: –E1 = K * 20 * F2. •Energy of Ti with DVS: –E2 = K * 20 * (F/2)2.•Clearly, E2 = (E1)/4Time taken = t1 (say)Time taken = t2 = 2 * t1Therefore, if we reduce the frequency we save energy but, we spend more time in performing the same computationCprE 458/558: Real-Time Systems (G. Manimaran) 12Energy-Time TradeoffsTimeEnergy Savings10204060CprE 458/558: Real-Time Systems (G. Manimaran) 13Simple DVS scheme handling RT-task•Consider a real-time task T1 = (20, 30)•Applying the simple DVS scheme discussed earlier:–If there is peak load T1 runs at maximum frequency (F) and meets the deadline with no energy savings–It there is less load T1 runs at half the maximum frequency (F/2) and completes at time = 40 thereby missing its deadlineCprE 458/558: Real-Time Systems (G. Manimaran) 14Simple DVS scheme handling RT-task20@FFrequencyF20 30time20@(F/2)FrequencyF20F/240 timeNo DVSDVS: Low workloadInference:DVS cannot be blindly applied to real-time embedded systemsCprE 458/558: Real-Time Systems (G. Manimaran) 15Energy aware scheduling in RT SystemsObjectives:Minimizing energy consumptionMeeting the deadlines.CprE 458/558: Real-Time Systems (G. Manimaran) 16Energy aware RTS Techniques•OS Level Energy Management–The operating voltage/frequency decisions are made at scheduling phase–The operating voltage is decided at every task release and task completion–These schemes are also known as Inter-task schemes•Compiler Level Energy Management–The operating voltage/frequency decisions are made at the compile time–The operating voltage is decided at the basic block level–These schemes are also known as Intra-task schemesCprE 458/558: Real-Time Systems (G. Manimaran) 17Real Time - DVS schemesThe RT-DVS algorithms can be broadly classified based on the granularity at which voltage scheduling is performed as follows:T1T2T3Inter-taskInter-task DVS scheme: Voltage scheduling is DVS scheme: Voltage scheduling is done on a task by task basis.done on a task by task basis.Intra-taskIntra-task DVS scheme: